1. Field of the Invention
The present invention relates to an improvement of a noble metal tip provided at a spark discharge gap. Such a tip may be used with a spark plug for an internal combustion engine. The present invention also relates to a method of manufacturing the same which improves heat resistance and durability.
2. Description of the Prior Art
A spark plug for an internal combustion engine has a center electrode and an earth electrode which face each other and produce a spark discharge when a high voltage is applied between the electrodes. Discharge tips composed of noble metals are mounted respectively on sections of the pair of electrode members facing each other to define a gap for producing spark discharge between the tips.
Conventionally, in order to prolong the life of such a spark plug, the tip structure has included a thermal stress relieving layer joined between a layer made of a discharge member and each electrode as disclosed, for example, in Japanese Patent Laid-open No. 60-262374.
However, the life of the product cannot be effectively prolonged simply by including the stress relieving layer if it is joined to the electrode by resistance welding.
When such a composite tip is resistance-welded to the electrode, welding current generates heat at the interface between the discharge layer and the stress relieving layer and thermal deformation is caused due to the heat and welding pressure. This deformation appears as an expansion in the radial direction of the tip, particularly at the interface between the discharge layer and stress relieving layer.
FIG. 5 shows a sectional structure of an earth electrode 14 of a spark plug having a composite tip 13 including a discharge layer 11 and a thermal stress relieving layer 12. In the Figure, composite tip 13, after it has been joined to earth electrode 14 by resistance welding, becomes generally trapezoidal in sectional shape due to the expansion of stress relieving layer 12. Alternatively, earth electrode 14 contacts the periphery of the interface between discharge layer 11 and stress relieving layer 12, due to a remarkable expansion of stress relieving layer 12. If the composite tip has a tapered sectional shape even before welding, as shown in the aforementioned Laid-open document, the deformation becomes even more pronounced after welding.
Further, even if the sectional tapered shape of the composite member is turned up side down as compared to the above example, the difference of size between the discharge layer and stress relieving layer is about 0.05 mm, which hardly compensates for the thermal deformation in the radial direction during resistance welding of the tip.
When a tip with the trapezoidal shape is used, discharge layer 11 becomes thin as a result of spark consumption over a long period of time and spark discharge is then generated from the periphery of stress relieving layer 12. Therefore, stress relieving layer 12 is consumed. Also, stress relieving layer 12 is directly exposed to high temperatures and the oxidizing atmosphere of the combustion chamber of the internal combustion engine, thereby advancing oxidation and corrosion thereof. Spark consumption, oxidation and corrosion of stress relieving layer 12 damage its thermal stress relieving function and causes discharge layer 11 to fail, shortening the life of the spark plug.
Accordingly, it is an object of the present invention to overcome the aforementioned problems by providing a spark electrode tip such as for a spark plug for an internal combustion engine and a method of manufacturing the same having a stress relieving layer joined to a discharge layer and also resistance welded to an electrode, yet which prolongs the life of the tip and is reliable.